Digital communication system infrastructures are known to include transcoders for compressing and encoding information signals prior to transmission from a base site to a mobile communication unit. The transcoders are similarly used for decoding and expanding information signals received from the communication unit prior to forwarding the information signals to a target unit, such as another mobile communication unit or a public switched telephone network (PSTN) subscriber. Such transcoders typically include an audio automatic gain control (AGC) circuit and a speech encoder in the transmit portion of the transcoder. The audio AGC circuit converts a varying amplitude input signal (e.g., a PSTN signal) into a substantially constant amplitude output signal for further processing by the speech encoder.
Generally, the audio AGC circuit applies a variable gain to its input signal based on the average amplitude level of the input signal with respect to two thresholds. When the average input signal level is greater than a first threshold, but lower than a second threshold, the audio AGC circuit applies a positive gain (amplifies) to the input signal in an attempt to raise the average AGC output signal level to the second threshold. Similarly, when the average input signal level is greater than the first threshold and greater than the second threshold, the audio AGC circuit applies a negative gain (attenuates) to the input signal in an attempt to lower the average AGC output signal level to the second threshold. In both of the above situations, the audio AGC circuit assumes that audio information is primarily responsible for providing the energy that raises the average input signal level above the first threshold. When the average input signal level is less than the first threshold, the audio AGC circuit presumes that the input signal primarily contains noise and simply passes the input signal without performing gain adjustments.
A problem arises due to the AGC circuit's assumption that audio energy is responsible for raising the average amplitude of the input signal above the first threshold. When a PSTN subscriber is conversing with a mobile communication unit, the telephone line can, on occasion, become exceedingly noisy to the extent that the average amplitude of the input signal is between the first and second threshold when the PSTN subscriber is silent (i.e., not talking). During this silent period, the audio AGC circuit still applies a positive gain to the input signal in an attempt to raise the average AGC output signal level to the second threshold, thereby effectively amplifying the noise. The output of the audio AGC circuit is applied to a speech encoder that detects the presence, or absence, of speech and generates a silence message when speech is absent. The silence message instructs the mobile unit to generate background noise in proportion to the AGC circuits output signal level. Thus, when the noise content of the AGC circuits output signal is excessive, the background noise generated at the mobile unit is also excessive, and sometimes annoying to a user of the mobile unit.
Therefore, a need exists for a method and apparatus for mitigating noise in an output signal of an audio AGC circuit that prevents the AGC circuits output signal from containing excessive noise during silent speaking periods.